You need to scale the spline itself, yes. You can do it in the editor, in the level or with script. Think of the Spline Component as of a data set, a clever container. When you add Spline Mesh Component (the cylinder), you read the spline’s data and apply it to the mesh.
You are already doing it to the Location and Tangent, now you’d need to incorporate the Scale as well:
You will only need 2 axis, because the stretching is already done with the node you’ve got up there.
Here’s an example of someone else doing it:
Note the +1 indexing, just like location and tangent. Do note that since you flip the orientation to Z, you will need to read the spline’s XY.